The present invention relates generally to motor control and deals more specifically with apparatus and a related method to compensate for torque ripple in a permanent magnet electric motor.
The use of a permanent magnet motor is generally well known with typical motor system applications including a combination of a permanent magnet alternator and an inverter drive whose output driving signal is synchronized to the rotor position. Such a motor is known in the art as a brushless DC drive motor and its configuration is somewhat similar to a conventional commutator type DC motor having a separately excited field winding. It is also known to excite the motor with an inverter drive output signal having a simple rectangular pulse waveform to cause the motor to operate at a speed which is substantially proportional to the magnitude of the DC voltage from which the inverter operates. The torque produced will be generally proportional to the DC current. It is further known to excite the motor with a pulse width modulated (PWM) inverter drive voltage to replicate the performance of a variable DC voltage driven motor.
The class of conventional brushless DC motors have advantages and disadvantages compared to the conventional commutator type DC electric motor, particularly when comparing the torque ripple and noise is that is generated. The conventional brushless DC motor produces large torque ripple which is due primarily to the relatively small number of phases driving the electric motor compared to the relatively large number of commutator bars on a conventional commutator type DC electric motor. The noise produced by a conventional brushless DC electric motor tends to be high as a result of the high torque ripple and also due to the relatively quick transitions of the phase commutation and pulse width modulation (PWM) steps in the driving voltage waveform.
It is desirable to reduce the torque ripple and noise generated by an electric motor particularly in applications wherein generated noise is of a great concern. Such an application might be for example, the propulsion of an undersea vehicle. Of particular importance as a source of noise is torque ripple which has a direct and difficult to interrupt path for transmission to the water through which the undersea vehicle passes.
One known approach to reduce torque ripple and noise in a brushless DC motor is to create a field flux and an armature current both of which are distributed approximately as a sinusoidal function of an angle and to then operate the inverter drive and filter the output of the drive in such a way to create an applied drive voltage which is substantially a sinusoidal function of time. The performance of a brushless DC motor approximates that of a AC electric motor with this approach. Although the torque ripple and noise are reduced, the approach is generally unsatisfactory due to the increase in size of the electric motor and the reduction in the power output achieved. In addition, the above method to reduce torque ripple and noise is somewhat limited due to the difficulty, complexity and often inability to approximate the desired waveforms.
It is a general aim therefore of the present invention to provide a permanent magnet electric motor and a solid state power inverter combination to precisely control output torque and to minimize noise generation.
It is a further aim of the present invention to permit the selection of motor magnetics based on high torque production rather than minimum noise generation.
It is a further aim of the present invention to provide noise reduction by controlling the current waveform driving the electric motor.